The present invention generally relates to cardiac therapy, and, more particularly, the present invention is concerned with cardiac therapies involving controlled delivery of electrical stimulations to a heart for treatment of atrial arrhythmias and an apparatus for delivering such therapies.
Cardiac arrhythmias can generally be thought of as disturbances of the normal rhythm of the heart muscle. Cardiac arrhythmias are broadly divided into two major categories, bradyarrhythmia and tachyarrhythmia. Tachyarrhythmia can be broadly defined as an abnormally rapid heart (e.g., over 100 beats/minute, at rest), and bradyarrhythmia can be broadly defined as an abnormally slow heart (e.g., less than 50 beats/minute). Tachyarrhythmias are further subdivided into two major subcategories, namely, tachycardia and fibrillation. Tachycardia is a condition in which the electrical activity and rhythms of the heart are rapid, but organized. Fibrillation is a condition in which the electrical activity and rhythm of the heart are rapid, chaotic, and disorganized. Tachycardia and fibrillation are further classified according to their location within the heart, namely, either atrial or ventricular. In general, atrial arrhythmias are non-life threatening, chronic conditions, because the atria (upper chambers of the heart) are only responsible for aiding the movement of blood into the ventricles (lower chambers of the heart), whereas ventricular arrhythmias are life-threatening, acute events, because the heart's ability to pump blood to the rest of the body is impaired if the ventricles become arrhythmic. This invention is particularly concerned with treatment of atrial fibrillation.
Various types of implantable cardiac stimulation devices are presently available and used for delivering various types of cardiac stimulation therapy in the treatment of cardiac arrhythmias. The two most common types which are in widespread use are pacemakers and implantable cardioverter defibrillators (ICDs). Pacemakers generally produce relatively low voltage pacing pulses which are delivered to the patient's heart through low voltage, bipolar pacing leads, generally across spaced apart ring and tip electrodes thereof which are of opposite polarity. These pacing pulses assist the natural pacing function of the heart in order to prevent bradycardia.
On the other hand, ICDs are sophisticated medical devices which are surgically implanted (abdominally or pectorally) in a patient to monitor the cardiac activity of the patient's heart, and to deliver electrical stimulation as required to correct cardiac arrhythmias which occur due to disturbances in the normal pattern of electrical conduction within the heart muscle. In general, an ICD continuously monitors the heart activity of the patient in whom the device is implanted by analyzing electrical signals, known as electrograms (EGMs), detected by endocardial (intracardiac) sensing electrodes positioned in the right ventricular apex and/or right atrium of the patient's heart, or elsewhere in the heart. More particularly, contemporary ICDs include waveform digitization circuitry which digitizes the analog EGM produced by the sensing electrodes, and a microprocessor and associated peripheral integrated circuits (ICs) which analyze the digitized EGM in accordance with a diagnostic algorithm implemented by software stored in the microprocessor. Contemporary ICDs are generally capable of diagnosing the various types of cardiac arrhythmias discussed above, and then delivering the appropriate electrical stimulation/therapy to the patient's heart, in accordance with a therapy delivery algorithm also implemented in software stored in the microprocessor, to thereby correct or terminate the diagnosed arrhythmias. Typical electrical stimulus delivery means used in ICDs involve an energy storage device, e.g., a capacitor, connected to a shock delivering electrode or electrodes. Contemporary ICDs are capable of delivering various types or levels of electrical therapy. U.S. Pat. No. 5,545,189 provides a representative background discussion of these and other details of conventional ICDs, and the disclosure of this patent is herein incorporated by reference.
In the treatment of a chronic cardiac condition, such as atrial arrhythmias, a challenge posed is that the patient typically is conscious and can potentially perceive any programmed electrical stimulation treatment being performed on his/her heart. Namely, one known method of electrical shock therapy for treating atrial (or ventricular) arrhythmia is to deliver a single burst of a relatively large amount of electrical current through the fibrillating heart of a patient. For a given atrial fibrillation episode, the minimum amount of energy required to defibrillate a patient's atrium is known as the atrial defibrillation threshold (ADFT). Generally speaking, the degree of pain, discomfort and trauma caused to the conscious patient receiving electrical stimulation as the mode of therapy for a cardiac fibrillation generally will be a direct function of the amount of electrical energy delivered to the patient's heart to terminate a given fibrillation episode.
Therefore, it is desirable that the energy levels of electrical stimulating shocks delivered by an implantable atrial defibrillator be reduced as much as possible, and ideally to below the pain threshold of the patient. Although the sensitivity to a electrical stimulus can vary from patient to patient in cardiac therapy, a current goal in the field of cardiac medicine is to reduce atrial defibrillation thresholds (ADFTs) to less than 1.0 joule, and more preferably, below 0.5 joule, to thereby reduce the required energy level of the defibrillation shocks to below the conscious perception levels of the vast majority of patients.
The electrical current and voltage requirements for conducting cardiac pacing therapy are relatively nominal in comparison to ADFTs. Consequently, the effects of pacing on atrial fibrillation has been the subject of several prior studies. However, previously reported studies of using local pacing alone to terminate atrial fibrillation have not indicated success. For instance, M. Allessie, et al., Circulation, vol. 84, No. 4, October 1991, pp. 1689-1697 and C. Kirchoff, et al., Circulation, vol. 88, No. 2, August 1993, pp. 736-749, describe use of rapid pacing in conscious dogs at a single atrial site at a rate faster than the atrial fibrillation cycle length to achieve a limited local capture but without termination of atrial fibrillation. These prior researchers demonstrated that during atrial fibrillation, a short and variable excitable gap occurs after local tissue emerges from the local refractory period when the cardiac tissue can be easily excited by a delivered pulse to achieve local capture before the next fibrillatory wavefront comes close enough to activate the area again.
If the cardiac tissue was homogenous, the pulsing from the one site should eventually entrain all available atrial tissue to extinguish all fibrillatory wavelets. However, this does not happen because atrial cardiac tissue is not homogenous. As documented in the field, electrophysiological properties such as conduction velocity, excitability, and refractory period have spatial inhomogeneity. E.g., see M. Wijffels, et al., Circulation, vol. 92, No. 7, October 1995, pp. 1954-1968. Thus, when a certain atrial region is paced at a certain rate other atrial regions with longer refractory periods cannot follow the higher pacing rate in a 1:1 manner. This results in conduction blocks. The atrial regions with longer refractory periods will continue at a slower rate than the rate of entrainment. The resulting asynchrony in activation will perpetuate fibrillation. Accordingly, these types of atrial inhomogeneities have permitted only a very limited area of capture to be achieved by prior uses of a single pacing site.
Although not directed to atrial defibrillation therapy per se, U.S. Pat. No. 5,161,528 teaches a method and apparatus for defibrillating a mammal with reduced energy requirements in which the heart's fibrillation cycle length is determined and then multiple sub-threshold bursts of electrical current are administered to the mammal with the burst intervals based as a percentage of the heart's fibrillation cycle. Preferably, the timing of successive bursts is set to be about 75% to 85% of the fibrillation cycle length. The sub-threshold shocks are insufficient by themselves to terminate depolarization wave propagation, but can be used to alter the timing of the depolarization wavefront along its reentrant pathways and thereby constrain the depolarization wavefront. While U.S. Pat. No. 5,161,528 broadly suggests use of a plurality of electrodes to concurrently deliver shocks through multiple different pathways, at the same or different times, the patent clarifies in the experiments described therein how the ventricular defibrillation is achieved using a determination of an average fibrillation cycle length value derived from a plurality of electrocardiogram measurements, and that the calculated average fibrillation cycle length is used as the basis for setting the electrical burst rate applied. The ventricular defibrillation therapy taught by U.S. Pat. No. 5,161,528 is not translatable to atrial defibrillation because the shocks delivered pursuant to the therapy of the &gt;528 patent reference require relatively large amounts of electrical energy, viz. 2.7 joules and even much higher, which is well outside the above-identified acceptable comfort zone of a typical conscious chronic patient in need of atrial defibrillation.
From the foregoing, it can be appreciated that there presently exists a need for a modality of delivering cardiac therapy that reduces atrial defibrillation thresholds to eliminate or at least significantly reduce the pain and discomfort to a patient undergoing atrial defibrillation treatment.
It is another object of this invention to provide a method for terminating atrial fibrillation or at least to improve atrial defibrillation efficacy by bringing large regions of atrial tissue into phase-lock via a regimen of pacing level pulses alone. The above and other objects, benefits and advantages are achieved by the present invention as described herein.